The present invention relates generally to semiconductor devices and, in particular, to devices with a pair of transistors having dual work function gate electrodes.
Various electronic devices, including digital-to-analog converters, operational amplifiers, and instrumentation amplifiers, require an accurate and stable voltage source to function properly. In particular, the voltage sources should be insensitive to changes in the ambient environment, such as changes in supply voltage or temperature.
Various techniques are known for providing a reference voltage source. In theory, a reference voltage can be generated using a pair of field-effect transistors (FETs) which are identical except for their gate electrode work functions. The different work functions results in corresponding different threshold voltages which can be used to provide the reference voltage.
FIG. 1 shows a simplified circuit, consisting of two metal-oxide-semiconductor (MOS) transistors T1 and T2 operating in the saturation region. The transistors T1, T2 are identical except for the values of their respective threshold voltages, which are achieved by providing the polysilicon gate electrodes of the transistors with different doping types. In other words, the gate electrode of one transistor, for example, the transistor T1, has a n-type conductivity, and the gate electrode of the second transistor T2 has a p-type conductivity. The threshold voltage of a MOS transistor can be expressed as             V      T        =                  V        FB            +              2        ⁢                  _ϕ          P                ⁢        _            +                                    2            ⁢                          ε              s                        ⁢                          N              d                        ⁢            2            ⁢                          (                              ϕ                p                            )                                                C          ox                      ,
where VFB is the flat band voltage of the MOS structure, xcfx86P is the bulk potential, Nd is the concentration of dopants in the channel, and Cox is the gate capacitance per unit area. The flat band voltage VFB is determined by the work function difference xcfx86MS between the gate material and semiconductor material in the channel region, and also by the charge residing at the interface states and within the gate oxide. The work function difference xcfx86MS corresponds to the difference between polysilicon and bulk Fermi levels. A polysilicon layer, used as the gate electrode, is heavily doped and, therefore, the Fermi level is effectively pinned at the conduction or valence band edges for N+ and P+ type polysilicon, respectively.
If the two transistors T1, T2 are formed using the same process flow such that the only difference between them is the polysilicon doping, the threshold voltage difference, VT1xe2x88x92VT2, would be identical to the work function difference between the bulk silicon, and N+ poly and P+ poly, respectively. In such a situation, the work function difference xcfx86MS of the N+ oxide-silicon and P+ oxide-silicon systems would differ by the value of the energy band gap of silicon (approximately 1.11 eV at room temperature). Accordingly, the threshold voltage difference also would be close to that value, regardless of the channel doping, gate oxide thickness and interface properties.
As shown in FIG. 1, drain D of each transistor is electrically coupled to its respective gate G. Thus, for each transistor T1, T2, the gate-to-source voltage VGS equals the drain-to-source voltage VDS. Therefore, both transistors operate in the saturation regime, and their drain currents can be expressed as             I      DS        =                  β        2            ⁢                        (                                    V              GS                        -                          V              T                                )                2            ⁢              xe2x80x83            ⁢      where                  β      =                        W          L                ⁢                  C          ox                ⁢        μ              ;  
W, L are the gate width and length respectively, xcexc is the carrier mobility in the channel, and Cox is the capacitance of gate oxide per unit area. Transistors such as those shown in FIG. 1 which are substantially identical except for the value of their respective threshold voltages are sometimes referred to as xcex2-identical transistors.
Also, in the circuit shown in FIG. 1, the drain currents ID1, ID2 of the transistors T1, T2 are substantially identical. Substantially identical drain currents can be achieved by using a current mirror in the drain of each transistor. The difference in the respective drain-to-source voltages xcex94VDS (=VDS1xe2x88x92VDS2) equals the difference in threshold voltages VT1xe2x88x92VT2. The voltage difference xcex94VDS is not dependent on the drain current or the supply voltage. The temperature dependence of xcex94VDS is controlled primarily by the temperature dependence of the silicon band gap, which is approximately 0.3 mV/K. Thus, the voltage difference xcex94VDS can be used as a relatively stable voltage source.
Various devices have been proposed using pairs of FETs to generate a reference voltage source. However, many of the proposed devices are difficult to implement using standard CMOS technology and fabrication flow processes. Moreover, in some designs for a reference voltage with a pair of xcex2-identical transistors, at least a portion of one of the polysilicon gates above the channel region of the transistor has a different conductivity type than a central portion of the gate. As a result, the central portion of the transistor gate is shorter than the channel length of the transistor. Such a design effectively results in two additional transistors in series with the central transistor which can lead to the output reference voltage being dependent on the drain current.
In general, techniques are described for fabricating a pair of transistors which more closely approximate ideal xcex2-identical transistors, in other words, a pair of transistors whose dimensions and electrical characteristics, other than their respective gate electrode work functions, are substantially similar. The techniques described in greater detail below can be incorporated into a standard CMOS process and can help avoid some of the problems discussed above. The techniques, however, are not limited to the formation of precisely xcex2-identical transistors or to the use of CMOS processes.
In one particular aspect, a semiconductor device includes a substrate of a first conductivity type and a pair of field effect transistors formed in the substrate. Each transistor includes source and drain regions of a second conductivity type opposite the first conductivity type and a channel region. An area extending from the source region to the drain region defines a length of the channel. Each transistor also includes a gate electrode disposed above the channel region. The gate electrode of a first one of the transistors is of the second conductivity type. A portion of the gate electrode of the second one of the transistors is of the first conductivity type and extends above the entire length of the channel of the second transistor. The lengths of the channels of the first and second transistors are substantially the same.
In another aspect, a semiconductor device includes a substrate of a first conductivity type and a pair of field effect transistors formed in the substrate. Each transistor includes source and drain regions of a second conductivity type opposite the first conductivity type and a channel region. An area extending from the source region to the drain region defines a length of the channel. Each transistor also includes a gate electrode disposed above the channel region and a field oxide region disposed between the gate electrode and the channel region. The gate electrode of a first one of the transistors has a first work function and includes dopants only of the first type of conductivity. The gate electrode of the second one of the transistors has a second work function and includes dopants only of the second type of conductivity. The lengths of the channels of the first and second transistors are substantially the same.
Such devices can be used to form, for example, a reference voltage source and can be incorporated into digital-to-analog converters, operational amplifiers, instrumentation amplifiers, and other electronic devices requiring an accurate and stable voltage source.
Techniques for fabricating such devices also are described below.
Some implementations include one or more of the following advantages. The technique allows CMOS technology to be used to fabricate a pair of FET transistors which are substantially identical except for the doping of their respective gates. The use of CMOS technology allows the formation of xcex2-identical transistors to be integrated easily into standard device and circuit fabrication processes with few, if any, modifications. Moreover, the present invention permits two transistors to be fabricated in a manner that more closely approximates ideal xcex2-identical transistors. The present invention can, therefore, provide a stable reference voltage source that exhibits reduced dependence on the drain current of the transistors. A reference voltage source also can be fabricated that exhibits less dependence on temperature.
Other features and advantages will be readily apparent from the following detailed description, the accompanying drawings and the claims.